Would you say they are a bigger job than Real Madrid?
Inter are a big club but the second biggest club in their City and third biggest in their country so they view themselves as the underdog, which Mourinho thrives on.
Explained, in a better way, by Gab Marcotti:
'The siege mentality works when you can think of yourself as a victim or an underdog. It's a staple for coaches everywhere, right down to Little League. “Gentlemen, it's us against the world ... how bad do you want it?” That approach may have worked at Inter, where the club saw themselves as outsiders to the Milan-Juventus hegemony, and at Chelsea, where they were the nouveau riche upstarts upsetting the social order. But this was Spain, and Real Madrid -- more than any other club -- represent the Establishment. Trying to recast Real as some kind of persecuted underdog simply didn't play. Many Real fans saw it as humiliating. Supporters of other clubs, particularly those that did have legitimate gripes, saw it as a joke.
(It's not a coincidence that Mourinho enjoys his strongest support among Real's “Ultras Sur.” They're a subset of the fan base who legitimately do consider themselves outsiders battling “the system.” Mourinho naturally resonates with them.)'
http://www.espnfc.com/blog/_/name/espnfcunited/id/5412?cc=5739
So why did Real Madrid employ him then?